Cross-linked elastomeric polyester laticies

ABSTRACT

A process for preparing laticies of cross-linked, elastomeric aliphatic polyesters comprising preparing a solution of the polyester in an organic solvent, emulsifying the polyester solution, stripping out the solvent and cross-linking the resulting polyester latex by heating with a peroxide. Upon coagulation, the cross-linked elastomer is obtained as a free-flowing powder.

BACKGROUND OF THE INVENTION

This invention relates to cross-linked elastomeric polyesters. Moreparticularly, this invention relates to a method for preparingcross-linked elastomeric polyesters in latex form.

Linear, high molecular weight, aliphatic polyesters may be prepared bythe polyesterification of aliphatic glycols with saturated dicarboxylicacids, or by the self-condensation of an hydroxy carboxylic acid. When asmall portion of the glycol or carboxylic acid monomer containsethylenic unsaturation, the resulting linear polyester contains alimited amount of ethylenic unsaturation. Polyester gums are generallyvulcanizable with peroxides, and can be cured to form tough,oil-resistant elastomers. The including of a limited degree ofunsaturation in the polyester serves to improve the rate ofcross-linking and to lower the level of peroxide needed to accomplishthe curing step. The preparation of polyesters as liquids or gums andsubsequent vulcanization with a peroxide to form cured rubber articlesor sheet stock is more fully described in U.S. Pat. No. 2,448,585.

The esterification or transesterification processes employed for thepreparation of aliphatic polyesters are generally run in bulk or insolvents at elevated temperatures and essentially in the absence ofwater. The resulting high molecular weight polymer is a heavy liquid orgum which on curing becomes a solid, rubbery mass. Cured elastomericpolyesters have not been obtainable in a finely-divided particulate formsuch as a free-flowing powder or as a dispersed suspension or latex.Esterification processes do not lend themselves to being carried out inaqueous emulsion, and subjecting a cured rubber to a mastication orgrinding process results in a non-uniform mixture of coarse rubberparticles, rather than a finely-divided free-flowing powder.

SUMMARY OF THE INVENTION

The present invention provides a process for the preparation of across-linked elastomeric polyester in latex form. The resultingpolyester latex may be coagulated to provide a powdered cross-linkedpolyester rubber.

The process of the instant invention comprises the steps of providing alinear, high molecular weight aliphatic polyester rubber optionallycontaining a limited amount of ethylenic unsaturation, dissolving thepolyester rubber in a suitable organic solvent, mixing the polymersolution with water containing a surfactant, passing the mixture througha homogenizer to provide an emulsion, stripping the organic solvent fromthe emulsion to provide a latex, and heating the latex with a peroxidecatalyst to cross-link the latex particles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polyesters suitable for the purposes of this invention are thelinear aliphatic polyesters described and set forth in U.S. Pat. No.2,448,585. In general, the elastomeric polyesters may be described ascopolymers of aliphatic dicarboxylic acids and glycols wherein fromabout 0 to about 20% of the glycol and/or the dicarboxylic acid isethylenically unsaturated. More particularly, the polyesters will bepolymers of C₂ to C₁₀ alkylene glycols and C₄ to C₁₀ alkylenedicarboxylic acids wherein from 0 to about 20% of the glycol monomercomponent and/or the dicarboxylic acid monomer component is,respectively, a C₄ to C₁₀ dihydroxy alkene or a C₄ to C₁₀ alkenyldicarboxylic acid. Examples of suitable alkylene glycols includeethylene glycol, propylene glycol, tetramethylene glycol, pentamethyleneglycol, hexamethylene glycol, decamethylene glycol, the position isomersthereof such as isopropylene glycol neopentyl glycol, dimethyltetramethylene glycol and the like, as well as mixtures thereof.Suitable dihydroxyalkenes include, for example, dihydroxybutene,dihydorxyhexene and the like. The alkylene dicarboxylic acids include,for example, succinic acid, adipic acid, sebacic acid and the like, aswell as mixtures thereof. The alkenylene dicarboxylic acids which may beemployed for the preparation of unsaturated polyesters include maleicacid, fumeric acid, itaconic acid, citraconic acid and the like. Thepolyesters may be readily produced by the processes set forth in thecited prior art, and the particular method of their preparation does notform a part of the instant invention.

In preparing a cross-linked polyester latex according to the practice ofthis invention, the polyester is first dissolved in a suitable organicsolvent. To be useful for the purposes of this invention, the organicsolvent must be immiscible with water, and be sufficiently volatile tobe readily stripped from an aqueous emulsion. Examples of suitablesolvents include chloroform, methylene chloride, benzene, toluene andthe like.

The solution will contain from about 5 to about 20 wt%, preferably fromabout 7.5 to about 12.5%, of the polyester. Concentrated solutions ofpolyesters are generally extremely viscous masses which flow and aredispersed only with great difficulty. The particular concentration to beemployed will be chosen to provide a moderately viscous, readilypumpable liquid. Low concentrations, and in particular concentrationsbelow about 5 wt%, are to be avoided in part to reduce the amount ofsolvent to be stripped from the emulsion in a subsequent step. Inpractice it has been found that for most combinations of solvents andpolyesters a concentration in the preferred range of 7.5 to 12.5 wt%provides a readily dispersable, easily handled liquid.

An emulsion of the polyester is then prepared by mixing the solution ofpolyester with approximately an equal volume of water containing asurfactant. Any of the variety of commerically available surfactantssuch as the sodium alkyl aryl sulfonates may be employed to stabilizethe emulsion. An amount of from about 0.5 to about 10 wt% based on theweight of polyester to be emulsified is generally sufficient to providea stable latex. The mixing is carried out under high shear conditions,preferably by passing the mixtue through a homogenizer to completelydisperse the organic phase and form an essentially uniform, oil-in-wateremulsion.

The solvent will then be stripped from the emulsion by heating themixture and distilling the solvent. The distillation may be accomplishedunder vacuum, if desired, to hasten solvent removal, and steam-strippingor sparging may also be preferred. The resulting solvent-free latexcomprises finely dispersed, uncross-linked polyester in aqueousemulsion.

Cross-linking of the polyester in latex form is accomplished by adding asuitable peroxide catalyst such as benzoyl peroxide, lauryl peroxide orthe like, and heating the mixture to effect decomposition of theperoxide and cross-linking of the polyester. The amount of peroxide willbe from about 0.5 to about 5 wt%, based upon the weight of polyester.Although the peroxide may be dispersed into emulsion prior to strippingor into the solvent-free, uncross-linked latex, it is generally morepractical to add the peroxide to the solution of the polyester prior toforming the initial emulsion. The solvent-stripping and cross-linkingcan then be accomplished by heating the emulsion to effect solventremoval, then continuing the heating of the solvent-free latex tocomplete the cross-linking reaction. The product, a cross-linkedpolyester latex, may be employed in forming blends with laticies ofother polymers, or may be coagulated or precipitated to provide across-linked polyester rubber as a finely-divided, free-flowing powder.

EXAMPLE 1. PREPARATION OF THE POLYESTER

A resin flask was charged with 70.1 g (0.48 m) of adipic acid, 4.8 g(0.078 m) of ethylene glycol, 29.5 g (0.39 m) of propylene glycol, 6.4 g(0.072 m) of 1,4 dihydroxybutene-2, and 0.24 g of zinc acetate catalyst.The mixture was heated at 140°-150° C. under nitrogen for approximatelyfive hours. The temperature was then raised to 200°-220° C. over a twohour period, and held at that temperature under a vacuum of 0.5 mm foran additional two hours. The mixture was then cooled to roomtemperature. The reaction mass was a crummy solid liquid having a nsp/cof 0.61 (25° C.) and a number average molecular weight of 18,5000.

EXAMPLE 2. PREPARATION OF A POLYESTER LATEX

A solution of 20 g of the polyester of Example 1 in 200 ml of methylenechloride was prepared and added to 200 ml of water containing 0.2 g ofan alkyl aryl sulfonate surfactant. The mixture was passed twice througha Manton Gaulin homogenizer to produce a stable emulsion. The emulsionwas heated with stirring to 90° C. to distill out the methylenechloride, resulting in a stable latex of uncross-linked polyesterelastomer.

The latex was coagulated with isopropanol, collected by filtration anddried to give a tacky solid gum. The gum was completely soluble intoluene, indicating that no gel formation has occurred.

EXAMPLE 3. PREPARATION OF A CROSS-LINKED POLYESTER LATEX

A solution of 20 g of the polyester of Example 1 and 0.4 g of benzoylperoxide in 200 ml of methylene chloride was prepared and emulsifiedwith 200 ml of water containing 0.2 g of an alkyl aryl sulfonatesurfactant as in Example 2. The emulsified mixture was heated withstirring to 90° C. to distill out the methylene chloride, then held at90° C. for about 17 hours to effect coss-linking of the polyester. Theproduct was a stable latex of cross-linked polyester.

The latex was coagulated with isopropanol, collected by filtration anddried to give a free-flowing powder. The powdered cross-linked elastomerhad a gel content of 91% as determined by toluene extraction.

EXAMPLE 4. PREPARATION OF A CROSS-LINKED SATURATED POLYESTER LATEX

A saturated polyester was prepared from 0.48 m (97 g) of sebacic acidand 0.44 m (33.5 g) of propylene glycol, using 0.24 g of zinc acetatecatalyst and employing the process of Example 1. A solution of 20 g ofthe polyester and 1.0 g of benzoyl peroxide in 200 ml of methylenechloride was emulsified, solvent stripped and cross-linked by heating at90° C. for 16 hours, by following the methods of Example 3. The productwas a stable latex of cross-linked, saturated polyester.

The cross-linked, saturated polyester was coagulated with isopropanol,collected by filtration and dried to give a free-flowing powder. Thepowdered cross-linked elastomer had a gel content of 71%, determined byextraction with toluene.

As is apparent from the Examples, stable cross-linked saturated (Example3) and unsaturated (Example 4) polyester laticies are readily producedby the process of this invention. The coagulation of the cross-linkedlaticies result in free-flowing powdered elastomers, completelydifferent in physical characteristics from the tack gum elastomers(Example 1). That the powdery nature of the products is not the resultof precipitation from a latex is demonstrated by Example 2, in which anuncross-linked latex produces a gummy solid upon coagulation.

The instant invention will thus be seen to be a process for preparingcross-linked polyester laticies and free-flowing powdered polyesterelastomers. In view of the well known hydrolytic instability ofpolyesters, it is surprising and unexpected that the instant process,which includes a step of prolonged heating in aqueous media, does notdestroy the polyester elastomers. The product latices are useful informing blends with latex resins such as SBR laticies and the like. Thefree-flowing powdered elastomeric polyesters are readily processable inmilling and in Banbury equipment, and may be employed in forming blendsand alloys by such methods.

It will be apparent to one skilled in the art that a variety ofstabilizers, processing acids and the like may be incorporated into thelaticies produced by the processes of this invention and that variousfurther modifications may be made without departing from the broadteachings of this invention, the scope of which is defined by thefollowing claims.

I claim:
 1. A process for preparing a cross-linked polyester latexconsisting ofA. Preparing a solution comprising from 95 to 80 parts byweight of a water-immiscible organic solvent, from 5 to 20 parts byweight of an aliphatic polyester elastomer and from 0.5 to 5 wt%, basedon polyester, of an organic peroxide; B. Mixing said solution with anequal volume of water containing from 1 to 10 wt%, based on polyester,of a surfactant; C. Passing said mixture through a homogenizer to forman emulsion; D. Heating said emulsion to remove said solvent; and E.Further heating said emulsion to cross-link said polyester.
 2. Across-linked polyester latex produced by the process consisting ofA.Preparing a solution comprising from 95 to 80 parts by weight of awater-immiscible organic solvent, from 5 to 20 parts by weight of analiphatic polyester elastomer and from 0.5 to 5 wt%, based on polyester,of an organic peroxide; B. Mixing said solution with an equal volume ofwater containing from 1 to 10 wt%, based on polyester, of a surfactant;C. Passing said mixture through a homogenizer to form an emulsion; D.Heating said emulsion to remove said solvent; and E. Further heatingsaid emulsion to cross-link said polyester.
 3. A powdered, cross-linkedpolyester produced by the process ofA. Preparing a solution comprisingfrom 95 to 80 parts by weight of a water-immiscible organic solvent,from 5 to 20 parts by weight of an aliphatic polyester elastomer andfrom 0.5 to 5 wt%, based on polyester, of an organic peroxide; B. Mixingsaid solution with an equal volume of water containing from 1 to 10 wt%,based on polyester, of a surfactant; C. Passing said mixture through ahomogenizer to form an emulsion; D. Heating said emulsion to remove saidsolvent; E. Further heating said emulsion to cross-link said polyester;and F. Coagulating said latex and drying the solid, powdery product.